Materials having magneto-optical properties are well known. Simple glasses such as borosilicate crown glass, for example, are known to rotate the plane of polarized light when placed in a magnetic field. The rotation achieved (θ) is proportional to the length of the light path, , the strength of the magnetic field B, and a magneto-optical material dependent parameter known as the Verdet constant K:θ=BK .The Verdet K constant may have dimensions of min·Oe−1·cm−1, for instance. The rotation of light is called the Faraday effect.
Borosilicate glass has a rather low Verdet constant, and thus manufacture of devices such as Faraday rotators, isolators, modulators, etc., would require either or both of a very long path length and a very strong magnetic field. High density lead-containing glasses such as the heavy flints have a Verdet constant some four times larger than borosilicate glass, but still too low for practical magneto-optical devices. Doping such glasses with elements with high magnetic moments such as terbium increases the Verdet constant, but the value is still lower than desired, and large aperture devices are especially impractical, due to the difficulty of establishing the necessary very strong magnetic field across the device. Thus, it would be most desirable to provide materials which are translucent and which have a high Verdet constant.
F. J. Sansalone, “Compact Optical Isolator,” Applied Optics, 10, No. 10 pp. 2329-2331 (October 1971) describes the use of crystalline TAG to produce a compact optical isolator. The magnetic field necessary for this small aperture device was able to be produced by rare earth button magnets. According to Sansalone, TAG has a Verdet constant which is an order of magnitude higher than lead glass. C. B. Rubenstein, et al., “Magneto-Optical Properties of Rare Earth (III) Aluminum Garnets,” J. App. Phys., 35 p. 3069-70 (1964), measured the Verdet constants of several rare earth aluminum garnets, and found TAG to have the highest Verdet constant of those tested. Thus it appears that TAG would be the material of choice for magneto-optical devices, and yet only small and sometimes thin film devices have been constructed.
In U.S. Pat. No. 5,245,689, TAG has been proposed as one of two epitaxially deposited garnet layers in a magneto-optical waveguide, although no devices employing TAG seem to have been created. U.S. Pat. No. 6,580,546 describes that Faraday rotators are activated by an electromagnet; the device also containing semi-hard magnetic materials to enhance latching and to decrease the drive current necessary to cause switching. While TAG is again mentioned, no device employing TAG appears to have been constructed. The same applies to U.S. Pat. No. 6,493,139, which discloses TAG as useful for optical switches.
The reason that TAG has not been used in practical magneto-optical devices of any size is the difficulty of providing single crystal materials. As indicated by Oliver et al. U.S. Pat. No. 6,144,188, polycrystalline garnet films may be prepared by chemical vapor deposition followed by annealing. However, these polycrystalline films do not share the optical transmission characteristics of single crystal material, and are thus useful only in certain applications. Rubenstein, in 1964, grew TAG crystals measuring 3 millimeters on a side by crystallization from lead oxyfluoride flux in platinum crucibles. The flux was removed from the solidified mass using nitric acid. The procedure uses toxic ingredients and has not been amenable to commercialization. Despite the fact that Rubenstein states that crystals measuring several centimeters on a side are grown by this technique, Sansalone, seven years later, described a Faraday rotator of TAG single crystal provided by Rubenstein, and bemoaned the fact that the longest crystalline rod was only 1 centimeter long. With this 1 centimeter TAG rod, and rare earth magnets, a rotation of about 31° was obtained at 6328 Å, and a full 45° rotation could be achieved at wavelengths shorter than about 5000 Å. Longer crystals could have achieved 45° rotation at longer wavelengths, but were apparently unavailable.
M. Geho et al., “Growth of terbium aluminum garnet (Tb3Al5O12; TAG) single crystals by the hybrid laser floating zone machine,” Journal of Crystal Growth 267, p. 188-193 (2004) discloses that TAG shows incongruent melting behavior, which prevent large size crystal growth. Instead of growing single crystals by conventional techniques, Geho used a special floating zone (“FZ”) method of crystal growth, by stacking alternating sheets of aluminum oxide and terbium oxide followed by sintering to produce a porous stack. This stack was then heated in a floating zone furnace having four CO2 lasers arranged radially around the rod shaped green body, and assisted by four quartz halogen lamps similarly arranged. However, while the length of the crystal does not appear to be limited in such a method, the crystal diameter is only 3 millimeters.
Thus, there exists a need for TAG magneto-optical devices in large aperture sizes, and with full rotation at all relevant wavelengths. There also exists a need for TAG precursors to form translucent or transparent TAG devices.